This invention relates to techniques for testing tires, and more particularly, relates to electronic circuitry which eliminates jitter from data signals without losing data.
The tire industry has long sought an automated method of testing the ability of tires to adhere to the road during emergency braking. As shown in FIG. 1, brake testing is normally performed by mounting test tires on an automobile and attaching a fifth wheel to the rear of the automobile. The fifth wheel includes a device for generating a fixed number of data signals for each revolution of the fifth wheel.
The automobile is accelerated to a constant speed as it approaches a conventional skid pad. At a predetermined point, the brakes are applied. As the brakes are applied, the frequency of the data signals produced by the fifth wheel is continuously monitored. The frequency is at a maximum as the automobile approaches the skid pad, and is reduced to zero when the automobile is fully stopped by the brakes. The number of data signals produced by the fifth wheel is counted between two selected frequencies, the higher frequency being slightly lower than the maximum as the automobile approaches the skid pad, and the lower frequency being slightly higher than zero. The count is proportional to the braking distance.
Unfortunately, the signals produced by the fifth wheel contain a substantial amount of jitter due to road irregularities, car bounce, wheel balance, gear errors and pick-up errors. In the past, the jitter has prevented the signals from being analyzed by a computer. The computer is unable to separate jitter from the data signals, and therefore produces erroneous results.
Various techniques have been devised to adapt computer analysis to brake testing, but each has proved to be insufficient. Monitoring the frequency of the data signals is difficult because the frequency varies over a wide range. For example, frequency determination of low-frequency signals with a microcomputer is usually done by timing each cycle. If this low frequency determination technique is used for higher frequency signals, the timing of each cycle becomes inaccurate. Averaging multiple cycles can improve the situation. However, for low frequency signals, this technique lengthens the sample time, and the lengthening reduces accuracy, especially when the frequency determination must be obtained from changing frequencies, such as in the brake testing of tires.
The known prior art also would not provide a solution to the foregoing problem. For example, U.S. Pat. No. 4,270,183 (Robinson, et al.) discloses a data dejittering apparatus employing a buffer register 18. The apparatus temporarily stores data in a buffer register which is normally maintained in a half full condition. This technique might operate satisfactorily when the data does not deviate too far from a nominal frequency, but would be incompatible in an application, such as tire testing, in which the data varies over a wide frequency range that approaches zero at the lower end of the range.
The prior art also includes a variety of phase locked loop circuits, such as the one shown in U.S. Pat. No. 4,380,742 (Hart), but these circuits also do not appear to provide a solution to the problem. The difficulty with such circuits is that under wide swings of frequency, they tend to lose data input signals temporarily. This is a condition which cannot be tolerated for the brake testing of tires, because every input signal must result in an output signal that is free of jitter.
The applicants have discovered that jitter can be eliminated from data signals produced by data signal generator without losing data by using a unique combination of circuitry. An output signal generator is provided for generating output signals substantially free of jitter. A digital number circuit responds to the data signals and output signals by generating a digital number that remains substantially constant if the frequency of the data signals equals the frequency of the output signals, varies monotonically in a first direction if the frequency of the data signals is greater than the frequency of the output signals and varies monotonically in a second direction opposite the first direction if the frequency of the data signals is less than the frequency of the output signals. The digital number signal is conditioned so that the frequency of the output signals tracks the frequency of the data signals and so that one output signal is produced for every data signal over a wide range of frequencies. If the capacity of the counter is exceeded, an indicator circuit gives an alarm. The operator then knows that data has been lost and that the test must be rerun using a counter of increased capacity.
Use of the foregoing techniques enables jitter to be removed from data signals without losing data. For applications having wide swings in frequency, such as the brake testing of tires, the techniques enable computer analysis of the data with a degree of accuracy and reliability previously unattainable.